Apparatus for dielectric heating



April 10, 1951 R. H. BLEIWITT APPARATUS FOR DIELECTRIC HEATING 2 Sheets-Sheet 1 Filed Sept. 8, 1948 INVENTOR. fia/wuo him/er 5L Ew/rr fwd ,4 Tram 5y April 10, 1951 R. H. BLEWITT 2,543,093

APPARATUS FOR DIELECTRIC HEATING Filed Sept. 8, 1948 2 Sheets-Sheet 2 INVENTOR. fa/v/zw fiQm/e 545m Arrow/5y Patented Apr. 10, 1951 APPARATUS FOR DIELECTRIC HEATING Ronald Harry Blewitt, Birmingham, England, as-

signor to Dunlop Tire and Rubber Corporation, Buffalo, N. Y., a corporation of New York Application September 8, 1948, Serial No. 48,189

In Great Britain September 11, 1947 2 Claims. (Cl. 219-47) This invention relates to improvements in apparatus for dielectric heating.

It is known to heat articles made of a dielectric material by inserting them in the working space between metal plate electrodes, in which space an electric field is generated by supplying electrical energy thereto to a potential alternating with high frequency e. g. a frequency between 5 and 350 megacycles per second, the apparatus being normally within a grounded cage for safety and other reasons. In unbalanced systems, i. e. those systems in which one electrode is permanently grounded while the potential of the other fluctuates, there will be a field, frequently referred to as a stray field, between the latter electrode and the grounded cage and heat will thus be developed in any dielectric material which lies in the field. Heat so developed represents a loss of power and to that extent the system is ineflicient. The loss may even be so great that the generator becomes overloaded without suflicient heat being generated in the material to be treated. This is particularly liable to occur where material is treated in a continuous manner by being passed through the electrode space on a conveyor since the conveyor will itself be a dielectric, and that part of the structure of the conveyor outside the working pace will thus be heated by the stray field.

My present invention provides an apparatus for subjecting material to the heating eiiect of a high frequency electric field in which the stray field is reduced and the loss of power, due to heating objects in that field, is correspondingly diminished.

According to the present invention an apparatus for subjecting material to the heating effect of a high frequency electric field between a live electrode and a grounded electrode comprise an auxiliary electrode insulated from and adjacent to the live electrode on the side remote from the grounded electrode, said auxiliary electrode being adapted to be grounded.

The space between the auxiliary electrode and the live electrode may either be free from dielectric material, except for any insulating spacing members necessary to locate the electrodes in position, or the space may be used as an auxiliary working space by using it for heating goods. By disposing an auxiliary grounded electrode adjacent to the live electrode on the side thereof remote from the grounded electrode, substantially the whole of the electric field outsidethe working space will be between the live electrode and the auxiliary electrode, and will thus not extend in any substantial degree through structural elements or other dielectric materials in which it is not desired to generate heat. The power losses due to thi cause hitherto eXperienced will be thus reduced and the power input used more advantageously. It has also been found that in many cases the occurrence of parasitic oscillations is avoided.

It is common in delectric heating apparatus to enclose the electrode system in a grounded case; where such is provided in apparatus according to the invention the apparatus then comprises a grounded cage, the electrode system Within the cage, electric leads for grounding the auxiliary electrode and the electrode bounding the Working space on the side remote from it and a further electric lead whereby the remaining electrode can be put into communication with a source of high frequency alternating electric energy. Further the apparatus may be adapted for carrying out the heat treatment of dielectric material in a continuous manner by also providing conveying means constructed of dielectric material for carrying the material through the working space. The stray field will then be between the live electrode and the cage, and the auxiliary electrode is placed substantially nearer to the live electrode than is any part of the cage. To prevent the auxiliary electrode acting as a reflector the the distance between the auxiliary electrode and the live electrode should be considerably less than M4 and in general less than A/20 where x is the Wavelength of the oscillations to be set up in the apparatus and may, for example, be of the order of 10-15 cms., which for oscillations of a frequency of 20 megacycles corresponds to i/l50 \/100. The minimum spacing is set by the spark discharge gap in air.

The electrodes may be in the form of sheet metal plates disposed parallel to each other in either a vertical or a horizontal plane or, for some purposes, it may be desirable to have the two electrodes defining the working space at an angle to one another, the auxiliary electrode being then parallel to the live electrode. The auxiliary electrode may also be a ring whose shape is the periphery of the live electrode, the ring being disposed immediately above the periphery of the live electrode. If the objects to be heated are cylindrical objects, all three electrodes may be rings of diameter greater than those of the objects, the three rings being arranged co-axially and conveying means being provided to pass the.

objects through the rings along this common axis.

Where the electrodes are plate electrodes, the conveyor may be an endless belt, e. g. of rubber, disposed to be driven around horizontal rollers outside the cage, the belt entering and leaving the cage through openings provided for the purpose so that the upper part of the belt passes through the working space parallel and adjacent to the lower electrode; a sheet of polystyrene or other insulating material having a negligible power factor may be disposed between the two to support the belt, and runners of wood or other dielectric material may be provided to support the edges of the conveyor outside the working space.

The auxiliary electrode may be fixed relative to the live electrode or it may be adjustable relative thereto within the limits indicated above.

Adjustment of its position provides a means of tuning the circuit. A co-axial feeder of usual type is provided at a suitable point at one side through which the electrical energy is supplied to the appropriate electrode, and by which the other electrodes may be grounded.

In an alternate form of the invention theregion between the live electrode and the auxiliary electrode is used as a working space by introducing dielectric material to be treated in that space. For example in an apparatus such as that described above employing an endless belt, the belt may be caused to pass between the live electrode and the auxiliary electrode during the upper part of its travel and between the live electrode and the lower grounded electrode during the lower part of its travel, i. e. when moving in the opposite direction; goods can then be conveyed in one direction for treatment in the one space while other goods are conveyed in the opposite direction for treatment in the other space.

The invention will now be described with reference to accompanying diagrammatic representations of apparatus embodying the invention. In these diagrams- Fig. 1 represents a perspective view of one form of apparatus with the front of the cage removed;

Fig. 2 represents a sectional elevation of an alternative form of the invention in which the space betweenthe live electrode and the auxiliary electrode is used for heating articles;

Fig. 3 represents a. sectional elevation of a further form of the invention in which the position of the auxiliary electrode is adjustable.

In each diagram like parts are indicated by like numerals.

Referring first to Fig. 1 the apparatus comprises a grounded cage 1 within which are two horizontal parallel plate electrodes 2 and 3 spaced apart to form between them a working space 4 and supported by means not shown in the diagram. The upper electrode is connected to central lead 5 of a co-axial feeder 6 by lead I which lead 5 is adapted to be connected to a source of high frequency alternating electric energy outside the cage 1. The cylindrical conducting sleeve 8 of feeder 6 is in electrical contact with the cage and also with the lower electrode 3 through lead 9, so that the latter is thus grounded. Two openings in and H are provided, one at each end of the'cage, to admit a horizontal endless rubber belt conveyor I2 supported and driven by rollers l3 outside the cage; the upper part of conveyor [2 passes through the working space 4, and the lower part passes beneath the electrode 3.

Immediately above electrode 2 is an auxiliary plate electrode I4 of substantially thesize and 4 shape of electrode 2 and supported on electrode 2 by insulating members l5, one at each corner. The electrode I4 is in electrical communication with the sleeve 8 of feeder 6 through lead I6 and is thus grounded. The distance between the electrode 2 and the auxiliary electrode l4 may, for example, be approximately 10 cms., for apparatus designed to operate with alternating electrical energy supplied to electrode M at a potential of 6 kilovolts and a frequency of 20-40 megacycles per second. This spacing is substantially less thank/4 and less even thanx/ 100. Under such conditions the separation of the auxiliary electrode I4 from the electrode 2 is such that sparking will not occur, nor standing waves be set up. The auxiliary .electrode is much closer to the live electrode 2 than the surrounding grounded cage I, so that the field between live electrode 2' and the cage I is reduced to small proportions; power losses through heating of the conveyor in passing through the regions between the edges of the electrodes and the walls of the cage are thus reduced to a correspondingly small value.

When the apparatus is in use electrode 2 is supplied with electrical energy of appropriate potential and frequency from a high frequency generator, and the conveyor 22 is set in motion by causing rollers i3 to rotate in the direction of the arrows. Articles to be heated, e. g. cut sheets of thermoplastic dielectric material which are to be warmed prior to shaping are placed on that part of the upper portion of conveyor l2 passing into the cage and thus travel through the working space 4 where they become warmed and then pass out of the cage on the conveyor at the far end. They are then removed from the conveyor and submitted to subsequent processing as desired. at

The apparatus illustrated in Fig. 2 includes a grounded cage I and parallel plate electrodes 2 and 3 as described with reference to Fig. 1, and an auxiliary electrode l4 disposed parallel to and above electrode 2. The electrode 14 is in electrical communication with the grounded sleeve 8 of feeder 6 and is preferably supported independently of electrode 2 by means not shown.

An endless rubber belt conveyor I2 is provided which is supported and driven in the direction indicated by arrows by rollers I3 outside thecage, but in this form of apparatus the lower portion of the endless conveyor passes through the working space 4 between the grounded electrode 3 and the live electrode 2, while the upper part of the conveyor passes through the space H between electrode 2 and the auxiliary electrodel l. Chutes l8 and I9 are provided by which articles to be treated can be respectively delivered onto the lower portion of conveyor 12 at the end thereof entering the cage, and removed at the other end beyond the cage after passing through working space 3, while chutes 20 and 2! enable other articles to be respectively delivered onto the end'of the upper part of the field between l-ive electrode -landgrounded' elec trode 3, or by delivery by chute 20 whereby they are passed through the space ii and become heated by virtue of the field between live electrode 2 and auxiliary electrode !4. The presence of electrode 14 ensures that the field outside the working spaces is reduced, even though space I! is used for heating purposes, and power losses through heating of the conveyor outside the working spaces are again avoided.

The apparatus of Fig. 3 comprises the grounded cage I and parallel plate electrodes 2 and 3 separated by the working space e and in electrical communication with elements 5 and 8 respectively of co-axial feeder '6 by leads 1 and 9 as described with reference to apparatus of Fig. 1. An endless conveyor belt 12 is also provided which is so supported and driven by rollers 13 outside the cage that the upper part thereof passes through the working space 4.

The auxiliary plate electrode 54 is of similar shape to electrode 2, and is suspended immediately above it by four wires 22, one end of each of which is attached to an insulator l5 projecting upwards from a corner of the electrode; the wires pass over idler pulleys 23 and are attached at their ends to a rotatable spindle 24. By rotating the spindle 24 the separator of electrodes l4 and 2 can be varied; since electrodes 2 and I4 together form a condenser in parallel with that formed by plates 2 and 3, movement of spindle 24 enables the effective capacity of the circuit of which these condensers form part to be varied and the circuit to be tuned. This method of adjustment is convenient when temporary modification to the tuning have to be made, for example if a batch of articles is to be treated of a difierent thickness of dielectric constant from a preceding batch, with consequent slight modification of the effective capacity of the condenser formed by electrodes 2 and 3. Useful variation in capacity can be made in this way without moving the auxiliary electrode 14 so close to electrode 2 that sparking may occur between the two, or without moving it so far away that the field between the electrode 2 and the cage is undesirably increased.

In the above description and in the claims the term dielectric material is used to mean a material which is an insulator having a significant power factor, as for example rubber, wood, porcelain, as distinct from conducting materials, e. g. metals, and air or materials such as polythene or polystyrene which although insulators, have a negligible power factor.

Having described my invention, I claim:

1. The apparatus of claim 2 in which said auxiliary electrode is spaced from the live elec: trode by a distance of less than M4, where is the normal wave length of the high frequency alternating current to be applied to the apparatus.

2. Apparatus for subjecting material to the heating effect of a high frequency electric field which comprises a pair of plate electrodes spaced to provide a working space between opposed faces of said electrodes, one of said electrodes being grounded and the other being live, a conveyor to pass material to be heated through said working space, a grounded cage of electrically conductive material surrounding said electrodes and an auxiliary grounded electrode within said cage insulated from said live electrode and having a face spaced from the face of said live electrode remote from said grounded electrode.

RONALD HARRY BLEWITT.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,226,871 Nicholas Dec. 31, 1940 2,259,318 Mouromtseii Oct. 14. 1941 2,303,983 Brown Dec. 1, 1942 2,370,624 Gillespie Mar. 6, 1945 2,433,842 Griffin Jan. 6, 1948 2,483,569 Baker Get. 4. 1949 FOREIGN PATENTS Number Country Date 558,064 Great Britain Dec. 17, 1943 OTHER REFERENCES ngaman et al.: Shielding of Dielectric Heating Installations, lectronics, Ma 19 pages 106-109, 1 

